What's the UK's favourite expression for plural you: you all, all of you, you guys etc.? I read that in the US people know where you are from depending on which way you say it.

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Depends on context. I don't think the average Brit uses one form over the other. It is not like the US which is basically the size of Europe, so can have much wider regional variation. You can hear all of these types at any time in British English by the same person:

"Omg, you all suck!"
"I'm gonna get all of you!"
"You guys at it again?"
"You lot are the worst!"
"You have to finish your coursework by tomorrow."

Unlike Americans, we never use certain forms, e.g. "Y'all". We also avoid the use of neologisms for really basic, short and frequently used words like pronouns.